Moreover, adverse effects of a warming climate are “tilted against many of the world's poorest regions” and likely to undermine development efforts and global development goals, says the study by the Potsdam Institute for Climate Impact Research and Climate Analytics, on behalf of the World Bank. The report, urges "further mitigation action as the best insurance against an uncertain future."

"A 4°C warmer world can, and must be, avoided – we need to hold warming below 2°C," said World Bank Group President Jim Yong Kim. "Lack of action on climate change threatens to make the world our children inherit a completely different world than we are living in today. Climate change is one of the single biggest challenges facing development, and we need to assume the moral responsibility to take action on behalf of future generations, especially the poorest."

A 4°C warmer world can, and must be, avoided – we need to hold warming below 2°C

The report says today’s climate could warm from the current global mean temperature of 0.8°C above pre-industrial levels, to as high as 4°C by 2100, even if countries fulfill current emissions-reduction pledges.

"This report reinforces the reality that today’s climate volatility affects everything we do," said Rachel Kyte, the Bank’s Vice President for Sustainable Development. "We will redouble our efforts to build adaptive capacity and resilience, as well as find solutions to the climate challenge."

The World Bank doubled lending for climate change adaptation last year and plans to step up efforts to support countries’ initiatives to mitigate carbon emissions and promote inclusive green growth and climate-smart development. Among other measures, the Bank administers the $7.2 billion Climate Investment Funds now operating in 48 countries and leveraging an additional $43 billion in clean investment and climate resilience.

Rising Sea Levels

The report says sea levels have been rising faster in the last two decades than previously, and this rise is being seen in many tropical regions of the world. This phenomenon is partly due to melting of the Greenland and Antarctic ice sheets; the rapid growth in melt area observed since the 1970s in Greenland’s ice sheet is a clear illustration of its increasing vulnerability. Arctic sea ice also reached a record minimum in September 2012. "There are indications that the greatest melt extent in the past 225 years has occurred in the last decade," says the report.

"It’s early yet but clearly some of the small island states and coastal communities are beginning to take a hard look at their options," said Erick Fernandes, co-lead of the Bank’s Global Expert Team on Climate Change Adaptation. "The need to adapt to climate change will increase as global population reaches 9 billion in 2050," he added.

Ocean Acidification

Coral reefs are acutely sensitive to changes in water temperature and acidity levels. The report warns that by the time the warming levels reach 1.4° C in 2030s, coral reefs may stop growing. This would be a result of oceans becoming more acidic as a result of higher CO2 concentrations. And with 2.4° C, coral reefs in several areas may actually start to dissolve. This is likely to have profound consequences for people who depend on them for food, income, tourism and shoreline protection.

Heat Extremes

A 4°C warmer world would also suffer more extreme heat waves, and these events will not be evenly distributed across the world, according to the report.

Sub-tropical Mediterranean, northern Africa, the Middle East, and the contiguous United States are likely to see monthly summer temperatures rise by more than 6°C. Temperatures of the warmest July between 2080-2100 in the Mediterranean are expected to approach 35°C – about 9°C warmer than the warmest July estimated for the present day. The warmest July month in the Sahara and the Middle East will see temperatures as high as 45°C, or 6-7°C above the warmest July simulated for the present day.

Lower agricultural yields

Hotter weather could in turn lower crop yields in a 4°C world—raising concerns about future food security. Field experiments have shown that crops are highly sensitive to temperatures above certain thresholds. One study cited in the report found that each “growing degree day” spent at a temperature of 30 degrees decreases yields by 1% under drought-free rain-fed conditions.

The report also says drought-affected areas would increase from 15.4% of global cropland today, to around 44% by 2100. The most severely affected regions in the next 30 to 90 years will likely be in southern Africa, the United States, southern Europe and Southeast Asia, says the report. In Africa, the report predicts 35% of cropland will become unsuitable for cultivation in a 5°C world.

Risks to Human Support Systems

The report identifies severe risks related to adverse impacts on water availability, particularly in northern and eastern Africa, the Middle East, and South Asia. River basins like the Ganges and the Nile are particularly vulnerable. In Amazonia, forest fires could as much double by 2050. The world could lose several habitats and species with a 4°C warming.

Non-linear impacts

As global warming approaches and exceeds 2°C, there is a risk of triggering nonlinear tipping elements. Examples include the disintegration of the West Antarctic ice sheet leading to more rapid sea-level rise, or large-scale Amazon dieback drastically affecting ecosystems, rivers, agriculture, energy production, and livelihoods. This would further add to 21st-century global warming and impact entire continents.

The projected 4°C warming simply must not be allowed to occur—the heat must be turned down. Only early, cooperative, international actions can make that happen.